Synthesis of the solution-processed wide band-gap chalcopyrite CuGa(S,Se)₂ thin film and its application to photovoltaics

A wide band-gap chalcopyrite CuGa(S,Se)₂ (CGSSe) thin film was synthesized via the simple solution based method of precursor solution coating and subsequent heat treatment processes. Various characterizations such as X-ray diffraction, UV–vis–NIR spectroscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy, Hall measurement, and dynamic secondary ion mass spectroscopy revealed a successful formation of the wide band-gap polycrystalline chalcopyrite film with nearly the same amounts of S and Se elements. The synthesized CGSSe film was applied as an absorber layer to the conventional thin film solar cell construction, which yielded an open circuit voltage (V_oc) of 0.52 V, a short circuit current density (J_sc) of 3.5 mA·cm^{− 2}, and an overall power conversion efficiency (PCE) of 1.0%. The photovoltage was lower than the expected value implying that there is a large voltage loss relative to the wide band-gap (~ 2.11 eV). It would be attributable to improper combination of the CGSSe absorber film and the CdS buffer layer in the conventional cell architecture, which may promote interface recombination due to an absence of a “spike” band alignment (i.e. conduction band offsets are slight positive values, 0.1 eV < ΔE_c < 0.4 eV) with the CGSSe absorber layer. Moreover, improvements in morphologies, such as enlarged grains and reduced pore sizes, need to be achieved for higher solar cell efficiency.